Effects of land use on trophic states and multi-taxonomic diversity in Japanese farm ponds

Farm ponds are among the most biodiverse anthropogenic freshwater habitats because of their small size, shallow water depth, and aquatic vegetation. Land-use changes, such as converting riparian vegetation to human use or changing the management practices of farm ponds, are assumed to be major factors that change such ecosystems from a clear-water state to a turbid state, leading to deterioration of water quality and biodiversity in such ponds. Using the database of a large-scale pond survey, we evaluated the effects of surrounding land use (landscape factors and modern pond management practices), fish abundance, and other environmental variables on total phosphorus concentration and taxonomic richness patterns of six biological indicators associated with changes in the trophic state. Local- and landscape-level vegetation structure associated with land use and total fish abundance were among the factors influencing the total phosphorus concentration of farm ponds, a main driver of trophic state changes. In addition, a transition from a clear-water state to a turbid state was associated with lower taxonomic richness of aquatic plants, macroinvertebrates, and adult Odonata, and a higher taxonomic richness of phytoplankton and fish. Based on these results, we discuss potential land-use and pond management strategies for conserving and/or restoring the water quality and biodiversity of farm ponds through maintenance of a clear-water state.